One First-Class Mail Forever stamp covers a standard letter weighing up to 1 ounce. A letter can weigh up to 3.5 ounces and still be mailed as a First-Class letter, but every ounce over the first needs additional postage. Over 3.5 ounces, USPS charges it as a large envelope (flat), up to 13 ounces.
The letter also has to be the right size and shape: rectangular, no larger than 6-1/8 × 11-1/2 inches and no thicker than 1/4 inch. This guide explains the weight steps, the size rules, and what happens when a letter is too heavy for one stamp.
How much weight does one First-Class stamp cover?
A single Forever stamp pays for a 1-ounce First-Class Mail letter. First-Class letter postage is set in steps of 1, 2, 3 and 3.5 ounces, so heavier letters need more postage for each step.
| Letter weight | Postage needed | Mail type |
|---|---|---|
| Up to 1 oz | 1 Forever stamp | Letter |
| Over 1 oz, up to 2 oz | Forever stamp + 1 additional ounce | Letter |
| Over 2 oz, up to 3 oz | Forever stamp + 2 additional ounces | Letter |
| Over 3 oz, up to 3.5 oz | Forever stamp + 3 additional ounces | Letter |
| Over 3.5 oz, up to 13 oz | Large envelope (flat) postage | Flat |
| Over 13 oz | Package postage | USPS Ground Advantage or Priority Mail |
You can cover the extra ounces with USPS Additional Ounce stamps, or with any mix of stamps that adds up to the right postage. For the current postage by weight, check the USPS price list or a postage scale at the Post Office.
How many pages can you mail with one stamp?
About four or five sheets of standard 20-lb printer paper in a regular #10 envelope weigh roughly 1 ounce, so that’s usually the most one stamp covers. Paper weight and envelope size vary, so weigh anything close to the limit.
Thickness can stop you before weight does. A letter can’t be thicker than 1/4 inch, and around 15 to 20 folded sheets get close to that, which is also about where the 3.5-ounce limit falls. Our guide to First-Class Mail facts and delivery times covers the service itself.
What size can a First-Class letter be?
To pay letter postage, a mailpiece has to meet every size rule in the USPS Domestic Mail Manual. If it’s bigger in any one dimension, it’s charged as a large envelope.
| Measurement | Letter minimum | Letter maximum | Large envelope (flat) maximum |
|---|---|---|---|
| Length | 5 in | 11-1/2 in | 15 in |
| Height | 3-1/2 in | 6-1/8 in | 12 in |
| Thickness | 0.007 in | 1/4 in | 3/4 in |
| Weight | No minimum | 3.5 oz | 13 oz |
Large envelopes have their own postage steps. Our guide on how many stamps you need for a large envelope explains the count for manila and other flat envelopes. To choose the envelope itself, our guide to big envelope sizes lists the common sizes and when each one makes sense.
What happens if your letter is over 3.5 ounces?
A letter over 3.5 ounces is charged large envelope (flat) prices, which are higher than letter prices. Flats can weigh up to 13 ounces, and anything heavier, or any envelope that’s rigid, lumpy or not rectangular, is charged as a package and sent as USPS Ground Advantage.
If you mail it with too little postage, USPS may return it to you or deliver it with postage due, which the recipient has to pay. If nobody’s home to pay, the carrier may leave a notice, and you can use USPS Redelivery to reschedule delivery.
A padded mailer is almost never a letter, because of its thickness. Our guide to how much it costs to ship a bubble mailer with USPS explains how USPS prices those as packages.
Which letters need extra postage even under 1 ounce?
Some letters can’t go through USPS sorting machines, so they’re charged a nonmachinable surcharge even if they weigh less than 1 ounce. Under the Domestic Mail Manual, a letter is nonmachinable if it:
- Is square or nearly square, or very long and narrow (a length-to-height ratio under 1.3 or over 2.5)
- Has the address parallel to the short side (a vertical envelope)
- Has clasps, strings, buttons or similar closures
- Contains items that make it uneven, such as pens, keys or coins
- Is too rigid to bend easily
- Is wrapped in plastic or has a non-paper outside (paper envelopes with windows are fine)
A square wedding or greeting card envelope is the most common example. One Forever stamp isn’t enough for these, even at 1 ounce or less.
Do Forever stamps work for heavier letters?
Yes. A Forever stamp is always worth the current 1-ounce First-Class letter price, whenever you bought it, so it covers the first ounce of any letter. You add postage for the extra ounces on top.
Two Forever stamps on a 2-ounce letter pay more than you need, since an additional ounce costs less than a first ounce, but USPS will deliver it. Our guide on whether Forever stamps expire explains how their value works.
How do you check your letter’s weight and postage?
A kitchen or postal scale that reads in tenths of an ounce is accurate enough for letters. Weigh the sealed envelope with everything inside, and measure its thickness if it’s stuffed.
- Weigh the sealed letter. At 1 ounce or less, one Forever stamp covers it.
- Check the shape. If it’s square, rigid or lumpy, add the nonmachinable surcharge.
- Check the size. Over 6-1/8 × 11-1/2 inches or 1/4 inch thick, it’s a large envelope.
- Add postage for each extra ounce, up to 3.5 ounces.
If you’re unsure, the clerk at the counter will weigh it, and our guide on whether the post office is open on weekends helps you plan a visit. Common postage questions are answered in our USPS frequently asked questions.
What if it’s too heavy for a letter?
Once a mailing is over 13 ounces, or isn’t flexible, it’s a package. USPS Ground Advantage is usually the lowest-cost choice for small packages, and Priority Mail is faster.
For packages, you can compare UPS and USPS rates in the ParcelPath rate calculator and print a label. ParcelPath is free, with no subscription or per-label fee, and offers up to 89% off USPS. Our shipping calculator hub compares carrier tools, our complete guide to shipping labels covers printing and attaching them, and our tips on how to ship for less and our roundup of shipping services and solutions help you choose a service.
Frequently Asked Questions
How heavy can a letter be for a First-Class stamp?
One First-Class Forever stamp covers a standard rectangular letter weighing up to 1 ounce. You can mail a letter up to 3.5 ounces as First-Class Mail, but each ounce over the first needs additional postage. Letters over 3.5 ounces are charged as large envelopes, and letters must also be no thicker than 1/4 inch.
How many sheets of paper can I mail with one stamp?
About four or five sheets of standard 20-lb printer paper in a regular #10 envelope weigh roughly 1 ounce, the most one Forever stamp covers. Heavier paper, cardstock, photos or a larger envelope add weight quickly. If the letter is close to 1 ounce, weigh it on a kitchen or postal scale before you mail it.
What happens if I put one stamp on a 2-ounce letter?
A 2-ounce letter needs a Forever stamp plus postage for one additional ounce. With only one stamp, USPS may return it to you for more postage or deliver it with postage due, which the recipient pays. Either way, it can be delayed, so weigh letters that feel thick and add the extra ounce.
Does a square envelope need extra postage?
Yes. Square envelopes can’t go through USPS letter-sorting machines, so they’re charged a nonmachinable surcharge even when they weigh less than 1 ounce. The same applies to vertical envelopes, rigid letters and envelopes with clasps, strings or lumpy contents. One Forever stamp alone isn’t enough for these letters.
What is the weight limit for a large envelope?
A First-Class large envelope, or flat, can weigh up to 13 ounces. It can be up to 15 inches long, 12 inches high and 3/4 inch thick, and it must be flexible and evenly thick. Anything heavier, larger, rigid or lumpy is charged as a package, usually sent by USPS Ground Advantage.
Sources
- USPS: First-Class Mail (letter and large envelope limits, nonmachinable letters)
- USPS Domestic Mail Manual 101: letter and flat dimensions, nonmachinable criteria
- USPS: USPS Ground Advantage
Part of our USPS Shipping Rates guide. Related: USPS Discounts, USPS Shipping Rates Calculator, USPS Shipping Calculator.
